A device using wide-gap semiconductor having a band gap more than 2 eV, represented by GaN-FET (Gallium Nitride-Field Effect Transistor) or SiC-JFET (Silicon Carbide-Junction Field Effect Transistor), has excellent characteristics such as high-speed switching, and low on-resistance, compared to a MOSFET using silicon. On the other hand, many devices exhibit a normally-on characteristic in which a threshold voltage is about −3 V, and a drain current flows even if a gate voltage is 0 V.
A device even using GaN or SiC and having normally-off characteristic has been developed. However, since the threshold voltage is low such as about 2 V, MOSFET using silicon cannot be replaced as it is.
FIG. 5 illustrates a circuit diagram of a driver circuit (inverter circuit) 30 having a conventional structure. FIG. 5 illustrates the case in which a conventional normally-off silicon MOSFET is used as switching elements 14 and 15. A MOSFET 14 on a high-side is turned on or off by a control circuit 8 based upon an input signal from an input control terminal 3, while a MOSFET 15 on a low-side is turned on or off by a control circuit 9 based upon an input signal from an input control terminal 4. As a result, a voltage between a voltage V1 supplied from a power source 12 and a voltage V2 (here, a ground potential) is outputted to an output terminal 23. Operating voltages of the control circuits 8 and 9 are supplied by a voltage VH supplied from an internal power source 13 and a voltage VL (here, a ground potential).
However, when the above-mentioned structure is applied to a normally-on device, and the normally-off MOSFETs 14 and 15 in FIG. 5 are replaced by normally-on devices, the normally-on device is in on-state when a control signal is not inputted to a gate terminal, such as when the driver circuit is started, or when the internal power source 13 in a control circuit 31 is shut down. As a result, the switching elements 14 and 15 are simultaneously in on-state, so that a so-called arm short in which large short-circuit current flows might be caused.
With respect to this problem, Patent Document 1 describes a method in which a gate drive circuit (driver circuit) includes a second power source that is connected to a ground-side of a main power source and that is established in conjunction with the main power source, a negative voltage supplied from the second power source and a gate-on voltage supplied from a control power source are switched by a control signal, and the switched one is outputted to a gate of a switching element. In Patent Document 1, when the control power source is shut down with the main power source being established, an off-voltage from the second power source is applied to the gate of the low-side switching element, whereby arm short can be prevented.